In recent years, pain research has made revolutionary advances at levels ranging from genes to behavior. However, one basic level of organization that has proved particularly resistant to analysis is the intrinsic circuitry underlying pain transmission and modulation within the first central relay area, the spinal/medullary dorsal horn. Recently a novel technique was developed that has made it possible for the first time to dissect the fine organization of the synaptic connectivity of closely spaced neural elements with extraordinary spatial resolution. This technique, scanning laser photostimulation, would be ideal for revealing for the first time the intrinsic organization of the dorsal horn circuitry involved in pain. However, the initial setting up of this technique is fairly expensive and time-consuming, and its use has so far been almost entirely restricted to cerebral cortex, primarily in the visual system. I request 2 years of support to set up and collect initial data with scanning laser photostimulation to study pain (nociceptive) circuitry in the dorsal horn. The application of this technique to the pain system will open up to analysis for the first time an entire field of investigation into plasticity of dorsal horn circuitry and allow the resolution of questions that have been debated since the start of the modern era of pain research and the proposal of the gate-control theory of pain modulation.